1. Field of the Invention
The present invention relates to a method for detecting indirectly those angles through which a metal sheet has been folded into portions, during the operation of folding the metal sheet by a folding machine comprising a punch and a die which are movable relative to each other.
The invention also relates to a device for detecting the folding angles of a metal sheet which have been once folded, during the operating of folding the metal sheet.
The present invention further relates to a method for folding of a metal sheet using the above-mentioned method for detecting angles.
2. Description of the Prior Art
Folding operations have been now affected by inserting a metal sheet to be folded between a punch and a die. The die has a depressed seat, and the punch penetrates the depressed seat of the die by their relative movements in a certain distance under control of a central control unit of a folding machine. The controlling carried out on the basis of parameters such as the type and the thickness of the metal sheet, the desired folding angle, etc. establishes the amount of relative movement of the punch to the die, after having positioned the metal sheet in stationary contact with the die.
As a result, the punch penetrates the metal sheet against the depressed seat of the die in a predetermined distance; thus it folds the metal sheet into portions in the seat. The precision of the folding angles obviously depends on how deep the punch penetrates the depressed seat. The deeper the penetration of the punch is, the greater the precision is.
The described folding machine does not allow precise folding angles to be obtained by a single folding operation. In fact, during its plastic deformation, the metal sheet retains a residual elastic restoring which, when the punch is separated from the die, causes an elastic return of the metal sheet with a consequent reduction in the previously achieved folding angle of the metal sheet itself. Even though this phenomenon is known and is taken into account when determining the setting of the folding parameters by means of suitable corrective coefficients, it is nevertheless impossible theoretically to predict the actual amount of the phenomenon and, therefore, completely to correct it. Therefore, if precise folding angles are required it is necessary, after folding, to perform measurement of the angle obtained and, possibly, to perform a second and corrective folding operation. It is clear that this fact results in slowing down of and complicating the productive operations as well as greater production costs.